Coal-fired power plants continue to produce a significant proportion of the electricity requirements for the United States. The combustion and gasification of coal is widely recognized as a significant environmental issue due to the potential release of hazardous pollutants. As a consequence, air quality standards continue to tighten. This results in the implementation of scrubbers for emissions control, most notably sulfur dioxide (SO2), from coal-fired power plants.
Wet scrubber technology with lime slurry/limestone is a proven and commercially established process for flue gas emissions control, particularly SO2 removal, from coal-fired power plants.
Wet scrubbers are usually designed with 80 to 95% efficiency of SO2 removal. However, facilities often use additives such as magnesium-enhanced lime or organic acids to improve process efficiency by 5 to 10%. This is particularly true in light of the market value of so-called SO2 “removal credits” and the potential for significant economic gain. However, the use of additives at the absorber may cause difficulties with the implementation and performance of downstream biological treatment systems.
For example, Flue Gas Desulfurization (FGD) process wastewater contains elevated levels of chlorides; significant concentrations of heavy metal contaminants such as chromium, mercury, and selenium; often high levels of nitrates; and a very high solids content that consists primarily of calcium sulfate, calcium carbonate, magnesium hydroxide, and fly ash.
Treatment of FGD wastewater is a significant need for utility operations. Physical/chemical treatment processes are typically used for neutralization and calcium sulfate desaturation, removal of some heavy metals, clarification and sludge thickening. However, conventional chemical precipitation techniques do not reliably eliminate heavy metal contaminants such as selenium and hexavalent chromium below outfall discharge limits established by newer, more stringent regulatory requirements. Nor do these current practices remove nitrogenous pollution.
FGD process wastewater is the focus of increasingly stringent effluent requirements, with outfall discharge standards (monthly average and daily maximum) typically established for:                pH        Total Suspended Solids (TSS)        Total Nitrogen (TN)        Heavy Metals including but not limited to Arsenic, Chromium, Copper, Mercury & Selenium        Sulfides.        
Selenium is an essential micronutrient for animals and bacteria. However, it becomes highly toxic when present above minute concentrations. The oxidized species of selenium, selenate (Se VI) and selenite (Se IV), are highly soluble and bioavailable, whereas reduced forms are insoluble and much less bioavailable. Regulatory limits for soluble selenium remain variable with targets ranging from 800 ug/L down to the U.S. national drinking water standard of 50 ug/L, frequently depending upon the discharge receiving water body.
Selenium exists in multiple valence states in the natural environment and the impact of selenium speciation on treatment efficiency is known. Notably, Selenium in the form of Selenite (Se IV; SeO3) can be removed with 65 to 85% efficiency using physical-chemical treatment approaches while Selenate (Se VI; SeO4) removal efficiency is limited to <10% with physical-chemical treatment.
It would therefore be helpful to provide an enhanced biological treatment approach to circumvent such problems, optimizing downstream removal of TN and heavy metals from FGD wastewater while maintaining SO2 removal efficiency at the absorber stage.